Surface mount technology has made it possible to densely populate both sides of a circuit board with semiconductor devices. Because leads of a surface mount integrated circuit (IC) package may be placed closer together than through-hole pins of a dual-in-line package (DIP), the component size of surface mount ICs has also shrunken down. These small packages have been termed small-outline IC packages and chip carriers. Chip carriers are typically used in applications that require large lead counts and employ a variety of mounting techniques, such as flatpacks, quad flatpacks (QFPs), J-leads, Gull-wing leads, leadless, and the more recently developed ball grid arrays (BGAs).
The ball grid array mounting technique uses rows and columns of closely positioned solder balls located on one side of the package as the outer leads of the integrated circuit. Ball grid array packages offer many advantages, including lower yield loss from bent leads and misregistration, higher throughput from greater placement tolerances and a more repeatable assembly process. However, a major concern with the ball grid array package is non-coplanarity of the solder balls which may translate to defective solder joints.
Non-coplanarity may be primarily due to two defects, solder ball non-uniformity or package warpage. When the tips of the solder balls are not coplanar or lie on a flat plane, the resultant lead solder joints may be weak or defective. Therefore, the ability to inspect and verify the ball grid array for non-planarity is crucial. However, because the solder balls are arranged in rows and columns, the profiles of the solder balls located behind the outer rows and columns are not visible.
The ball grid array package has heretofore been inspected in what the industry has termed the "dead bug mode", where the ball grid array faces up. When examined in the dead bug mode, the distance from a reference point to the tip of each solder ball may be measured. Then with a three-point algorithm, a theoretical three-point plane that the ball tips lie in may be computed. Using this theoretical plane, it is determined whether all the ball tips fall into this plane. However, the computation of the theoretical plane does not take into account the weight distribution of the device. Therefore, the actual weight distribution of the device may cause it to rest on a different set of ball tips than the ones used to compute the theoretical plane. The result is an inaccurate estimate of which solder balls will actually support the weight of the device and an erroneous determination of the coplanarity of the ball grid array.